This invention relates to an image forming apparatus which forms images by superposing a photosensitive sheet with one surface coated with microcapsules encapsulating a colorless dye and a light-curable agent which hardens by exposure to light and an image transfer sheet with one surface coated with a developing agent having a chromogenic effect on this colorless dye and pressing them together.
Japanese Patent Publication Tokkai No. 58-88739 discloses a method of forming images by using a photosensitive sheet. The photosensitive sheet used by this method is characterized as having a sheet-like base material coated with microcapsules encapsulating a colorless dye of a specified kind and a light-curable agent which hardens when exposed to light. If a photosensitive sheet like this is exposed to image-forming light such as reflected light from a document, only those of the microcapsules on the sheet which are exposed to light are selectively hardened and a latent image corresponding to the image carried by the light is thereby formed on the photosensitive sheet. If an image transfer sheet with one surface coated with a developing agent having a chromogenic effect on the colorless dye in the microcapsules is superposed on such a photosensitive sheet having a latent image formed thereon and if they are then pressed together, those of the microcapsules which have not been hardened rupture, causing the colorless dye encapsulated therein to flow out thereof and to react with the developing agent on the image transfer sheet to form a visible image corresponding to the latent image.
Inside an apparatus for forming an image by such a method, a photosensitive sheet wound up into the form of a roll is usually stored inside a container at a specified position and kept away from the reach of stray light. After the photosensitive sheet is pulled out of such a container and while it travels inside a light-shielding housing structure, its microcapsule-coated surface is exposed to image-forming light at an exposure position and the photosensitive sheet is thereafter transported to a compression device which serves as a visible image forming device. In the meantime, an image transfer sheet is transported from a paper supply section to this compression device where it is superposed on top of the photosensitive sheet with a latent image formed thereon and these two sheets are pressed together.
Thus, since a latent image is formed on such a photosensitive sheet by exposing its microcapsule-coated surface to image-forming light such that only those of the microcapsules exposed to light should be selectively hardened, it is essential that the microcapsule-coated surface of the photosensitive sheet be exposed only to the image-forming light. In other words, the photosensitive sheet must be completely shielded from external light other than the image-forming light when it is transported inside the apparatus. Inside an image forming apparatus of this kind, however, the photosensitive and image transfer sheets frequently cause a jam. Whenever there is a jam in a paper transporting route, the apparatus must be opened such that the situation can be remedied. The housing structure is also opened for the purpose of maintenance. When the apparatus is opened, however, external light penetrates the interior and a part of the photosensitive sheet which has not been exposed to image-forming light may be exposed to external light. Since this causes the hardening of all microcapsules on the photosensitive sheet inclusive of those at areas where no image is desired, the quality of the image formed thereafter on the image transfer sheet will be significantly affected adversely.